Evolution of the surface abundances of ³He and ⁷Li (top, dashed and solid lines respectively), of ¹⁴N and of the carbon isotopic ratio (middle, solid and dashed lines respectively), and of the helium-burning luminosity and total stellar mass (bottom, solid and dashed lines respectively) on the TP-AGB of the 1.25 M_⊙ model computed with thermohaline mixing only. The abscissa is the time since the first thermal pulse.